A variety of coating compositions is used for the painting of automobiles, including lacquers and enamels. In general, lacquers have relatively high percentages of solvent and contain high molecular weight polymers. By contrast, enamels are based on low molecular weight polymers which require lower concentrations of solvent. In order to achieve equivalent protection using enamels, the molecular weight of the polymers used in enamels is increased through crosslinking. Typically, acid catalyzed reactions with melamine formaldehyde resins as crosslinking agents are used, in combination with elevated temperatures, for example, of about 250.degree. F.
The use of enamel coatings in combination with appropriate crosslinking procedures provides finishes with outstanding appearance. However, the crosslinking can complicate the refinishing of a painted surface. Occasionally, the speed of automobile assembly lines is reduced due to mechanical or supply problems, resulting in unusually long exposure to the elevated temperatures used in curing the enamel coatings. This results in higher levels of crosslinking which do not effect the quality of finish itself, but, if refinishing is needed, either in the factory or afterwards, can interfere with adhesion of subsequently applied coatings. For this reason, in refinishing operations, the original finish is generally thoroughly sanded, to prevent failure of subsequently applied coatings.
With increasing concern over the disposition of the large quantities of organic solvent inherent in lacquer systems, the use of enamels in automobile manufacture has increased in recent years. A continuing need accordingly exists for enamel systems which, after curing, will exhibit adequate adhesion to subsequently applied coatings so that extensive sanding of the original finish is not necessary in refinishing operations.